fix assembly reference (#3646)

Author: dsyme

vsintegration/Vsix/VisualFSharpOpenSource/VisualFSharpOpenSource.csproj

@@ -324,8 +324,8 @@
       <SpecificVersion>False</SpecificVersion>
       <HintPath>$(FSharpSourcesRoot)\..\packages\Newtonsoft.Json.10.0.2\lib\net45\Newtonsoft.Json.dll</HintPath>
     </Reference>
-    <Reference Include="System.Collections.Immutable, Version=1.3.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
-      <HintPath>$(FSharpSourcesRoot)\..\packages\System.Collections.Immutable.$(SystemCollectionsImmutableVersion)\lib\portable-net45+win8+wp8+wpa81\System.Collections.Immutable.dll</HintPath>
+    <Reference Include="System.Collections.Immutable, Version=1.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
+      <HintPath>$(FSharpSourcesRoot)\..\packages\System.Collections.Immutable.1.3.1\lib\portable-net45+win8+wp8+wpa81\System.Collections.Immutable.dll</HintPath>
     </Reference>
   </ItemGroup>
   <Import Project="$(FSharpSourcesRoot)\..\vsintegration\src\FSharp.tools.targets" />